1. Field of the Invention
The invention relates to method and system for determining horizontal stresses from shear radial variation profiles, more particularly, the invention relates to method and system for determining horizontal stresses from shear radial variation profiles using optimal selection of radial positions, in the presence of noisy sonic data from the borehole sonic measurement.
2. Background Art
Detailed knowledge of geological formation stresses is desirable in the hydrocarbon production business, because formation stresses can affect the planning of well stimulation treatments to enhance hydrocarbon recovery as well as provide predictions of sanding and wellbore stability. In addition, formation stress information is useful in determining the long-term stability of the formation and thus helpful in determining the suitability of the formation as a repository for waste disposal. Accordingly, there is a growing demand in the industry for the estimations or determinations of formation stresses.
The formation stress state can be characterized by the magnitude and direction of the three principal stresses, vertical stress (overburden stress), maximum horizontal stress and minimum horizontal stress, see, for example, FIG. 1 illustrates the spatial alignment of the three principal stresses. Generally, the overburden stress yields the principal stress in the vertical direction. The overburden stress (SV) can be reliably obtained by integrating the formation mass density from the surface to the depth of interest. Estimating the other two principal stresses (SHmax and Shmin) in the horizontal plane is the remaining task necessary to fully characterize the formation stress state.
Existing techniques for estimating the maximum and minimum horizontal stresses are based on analyzing borehole breakouts and borehole pressure necessary to fracture the surrounding formation, respectively. For example, U.S. Pat. No. 5,236,040, issued to Venditto, et al., discloses a method for determining the minimum principal horizontal stress within a formation by using a wireline retrievable circumferential acoustic scanning tool during an open-hole micro-fracturing test. The method includes continuously pumping fracturing fluid into a borehole while scanning the internal surface of the borehole through an acoustic scanning tool, and when fracture detected by the acoustic scanning tool, recording the fracturing fluid pressure as the magnitude of the minimum horizontal pressure.
Both borehole break-outs and hydraulic fracturing are destructive techniques that rely on assumed failure models. For example, a borehole breakout analysis can be used only in the presence of a compressive-shear failure and assumed cohesive strength and friction angle in the Mohr-Coulomb failure envelope (Gough, D. I., and Bell, J. S., 1982, Stress orientations from borehole fractures with examples from Colorado, east Texas, and northern Canada: Canadian Journal of Earth Sciences, v. 19, no. 7, p. 1358-1370). However, the hydraulic fracturing technique for the estimation of SHmax requires a reliable knowledge of the rock in-situ tensile strength that is difficult to obtain.
Another approach for estimating formation stresses is using sonic log data. U.S. Pat. No. 5,838,633, issued to Sinha, discloses a method for estimating formation in-situ stress magnitude and nonlinear constants of the formation. This method includes generating two-frequency acoustic signals, receiving the signals at receiving transducers which are oriented at two orthogonal directions in a horizontal plane normal to the borehole axis, analyzing the flexural wave dispersions for dipole sources aligned parallel and perpendicular to the maximum far-field compressive stress direction and analyzing the Stoneley wave dispersion derived from a monopole source. The flexural and Stoneley wave velocities are determined as a function of frequency. In addition, the flexural wave velocities parallel and perpendicular to the far-field stress direction are determined and associated with stress induced formation anisotropy. In presence of formation and borehole stresses above and beyond those in an assumed isotropic reference state, the borehole flexural and Stoneley wave velocity dispersions are also functions of the formation stresses and nonlinear constants. The Stoneley and flexural wave velocities are defined for a hydrostatically loaded reference state. A change in the flexural and Stoneley wave velocities due to uniaxial stress is used to determine the stress magnitude and a plurality of nonlinear formation parameters.
U.S. Pat. No. 6,098,021, issued to Tang et al., discloses a method for estimating formation stresses by using a monopole and a cross dipole acoustic measurements. A radially polarized monopole guided shear-wave is generated in a borehole. It is then determined whether the shear-wave has split into two shear-waves. If it has, the difference in velocities between the two split shear-waves is used to determine the stress induced anisotropy around and near the borehole. The difference in velocities of cross-dipole shear-waves and the direction of the fast shear-wave are measured and are used to determine the magnitude of the maximum shear stress and the maximum stress orientation of the geologic formation. Furthermore, a method of determining the stress-velocity coupling coefficients from laboratory measurements as well as through field measurement calibration is disclosed. The effect of borehole pressure is taken into account, especially on the field measurement calibration method for determining the stress-velocity coupling coefficients.
Although the existing solutions provide various approaches for estimating formation stresses, accurate estimation of geological formation stresses is desirable in the hydrocarbon production business, because formation stress determination is considered critical for hydrocarbon production planning, as well as providing prediction of sanding and borehole stability. As a result, there is a growing demand in the art for accurate estimation or determination of both the two horizontal stresses.